1. Field of the Invention
This invention relates to musical tone synthesis and in particular is concerned with an improvement for producing tones having a time variant spectral content.
2. Description of the Prior Art
In various methods employed to generate new varieties of musical tone, the underlying technique is a means for varying the spectral components of tone as a function of time. Musical tone generators of the type in which a waveshape rich in harmonics is passed through a sliding formant filter to produce a time-varying tonal effect by altering the relative strength of the harmonics are well known in the musical tone generation art. In analog signal tone generators, the sliding formant filter is generally implemented as a high pass or a low pass filter in which the cutoff frequency may be varied with time to alter the harmonic content of the musical waveshape at the output of the filter. In digital tone generation systems the equivalent effect of a sliding formant filter can be achieved by scaling the harmonic coefficients that are used in a computational algorithm to obtain the successive amplitude point values of the musical waveshape. Digital musical tone generation of this type are described in U.S. Pat. No. 3,956,960 entitled "Formant Filtering In A Computor Organ" and in U.S. Pat. No. 4,085,644 entitled "Polyphonic Tone Synthesizer."
Frequency modulation techniques have been incorporated into musical tone generators to obtain waveshapes with time variant spectral components. These systems employ the known characteristic that the side bands of a frequency modulated carrier frequency form overtones when the fundamental frequency of the tone corresponds to the modulation frequency. The application of frequency modulation techniques is described in the article "The Synthesis of Complex Audio Spectra by Means of Frequency Modulation," by J. M. Chowning, J. Aud. Eng. Soc., Vol. 21, No. 7, September 1973, pp 526-534. Also in U.S. Pat. No. 4,018,121 there is described a digital system for implementing a frequency modulation system to generate unique musical sounds.
In U.S. Pat. No. 4,175,644 entitled "Musical Tone Generator With Time Variant Overtones" a tone generator is described which incorporates a frequency modulation calculation into the calculation of the master data set used in the musical tone generation system described in U.S. Pat. No. 4,085,644 entitled "Polyphonic Tone Synthesizer." In this system the address for reading out trigonometric sinusoid function values from a sinusoid table are changed as a function of time in a periodic or sinusoidal fashion. The effect is to produce a sequence of sinusoidal value from the table which correspond to a series of points on a frequency modulated carrier signal.